The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
As solutions for fossil fuel depletion, high oil prices, etc. are sought, interest in energy storage technology for efficient energy usage is rapidly increasing.
Metal air batteries are batteries wherein a metal such as lithium (Li), zinc (Zn), aluminum (Al), magnesium (Mg), iron (Fe), calcium (Ca), sodium (Na), or the like is used as a negative electrode, and oxygen in the air is used as a positive electrode active material.
Metal air batteries may unlimitedly use oxygen in the air, thus having excellent energy density, compared to other secondary batteries. Thereamong, lithium air batteries using lithium (Li) as a negative electrode have theoretical energy density of about 3500 Wh/kg which is about 10 times higher than lithium ion batteries.
Hereinafter, an operation mechanism of a lithium air battery is described referring to Formulas 1 and 2 below.
When a lithium air battery is discharged, lithium metal of a negative electrode is oxidized, and thus, lithium ions and electrons are generated. Such lithium ions migrate to a positive electrode through an electrolyte, and the electrons migrate to a positive electrode through an exterior conductor or a current collector. In regard to the positive electrode, oxygen is supplied from external air. The supplied oxygen is reduced by the electrons, thus forming Li2O2.
In an opposite direction to this, charging of a lithium air battery proceeds. In a positive electrode, a lithium compound is decomposed and thus oxygen is generated. In a negative electrode, reduction of lithium ions occurs.(Negative electrode):Li→Li++e−  [Formula 1](Positive electrode):O2+2Li++2e−→Li2O2  [Formula 2]
One task task of lithium air batteries is to address instability of batteries when charged under excessively high voltage during charge/discharge.
In a positive electrode of lithium air batteries, oxygen reduction reaction (ORR) occurs during charge, and oxygen evolution reaction (OER) occurs during discharge.
In current lithium air batteries, the speed of the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) is low. Accordingly, electrons do not actively migrate in a battery and thus over-voltage during charge/discharge is higher than a theoretical voltage, which causes low energy efficiency (low charge/discharge efficiency).
Hereinafter, conventional technologies related to a catalyst that is used in an electrode of secondary batteries including lithium air batteries are described.
Korean Patent No. 10-1308740 relates to a preparation method of an intermetallic compound-including carbon nanofiber which may be used as a catalyst of lithium secondary batteries. In this case, a fiber-web catalyst may be provided and thus electrons may rapidly migrate, compared to a conventional case in which a particle phase is used. In addition, carbon nanofiber may be provided through a simple method, i.e., electrospinning. However, although a fiber web state is provided, a surface area thereof is not sufficient and thus a catalyst is not remarkably activated.
Korean Patent Laid-Open Publication No. 10-2014-0058784 relates to a positive electrode catalyst using a binary alloy for lithium air batteries. Here, two transition metals are prepared into an alloy to provide a catalyst having high activity to oxygen reduction reaction and oxygen evolution reaction. However, since the catalyst is a particle-type catalyst, a surface area is not wide and diffusion of a reactant is not good. In addition, since the catalyst is an alloy, performance of the catalyst is not consistent.